Because "test_when_finished" in our test framework queues the
clean-up tasks to be done in a shell variable, it should not be
used inside a subshell. Add a mechanism to allow 'bash' to catch
such uses, and fix the ones that were found.
* jk/test-lint-forbid-when-finished-in-subshell:
test-lib-functions: detect test_when_finished in subshell
t7800: don't use test_config in a subshell
test-lib-functions: support "test_config -C <dir> ..."
t5801: don't use test_when_finished in a subshell
t7610: don't use test_config in a subshell